In recent years, the strength increase of automotive component parts and materials for use in automotive component parts is being pursued for the purpose of weight reduction of motor vehicles, and improvement in safety. With regard to steel sheet, which is a representative one of such materials, the rate of use of high-strength steel sheets is increasing. However, the high-strength steel sheet, generally, is high in strength, and hard, and therefore offers only a small degree of freedom in the forming in terms of press formability, and also is poor in the shape fixability of the pressed product (formed article), giving rise to problems of no-good dimensional accuracy, short service life of press dies, etc. While betterment of these problems is being pursued by improving materials, a technology generally called hot-work pressing, hot pressing or hot stamping is increasingly employed for the purpose of obtaining component parts with further increased strength and with good shape accuracy. Specifically, the technology is a hot process in which a steel sheet is soften by heating it to or above 800° C. (Ac3 point), and is rapidly cooled simultaneously with the press forming so as to obtain a very-high-strength component part. Besides, a cold processing-quenching technology in which the cold processing is followed by a quenching process as mentioned above so as to provide a high-strength component part has also come to be used as an industrial technology.
Industrial machines represented by motor vehicles need to have sufficient corrosion resistance in the environments of use. Therefore, component parts obtained by forming in a cold process a zinc or zinc alloy-plated steel sheet that is excellent in cost and corrosion resistance are sometimes used in such industrial machines.
Japanese Patent Application Publication No. 2001-353548 (JP-A-2001-353548) discloses a manufacturing method for a high-strength formed component part which secures the protection against corrosion and decarbonization, and lubrication performance by obtaining a zinc or zinc alloy layer of 5 μm to 30 μm by heating and cooling. Japanese Patent Application Publication No. 2003-73774 (JP-A-2003-73774) discloses a steel sheet for hot pressing which has a barrier layer for preventing evaporation of zinc during heating. Japanese Patent Application Publication No. 2003-126920 (JP-A-2003-126920) discloses a hot-pressing method for a zinc or zinc alloy plated steel sheet. Japanese Patent Application Publication No. 2003-126921 (JP-A-2003-126921) discloses a hot-pressed formed item that has an iron-zinc solid solution layer.
However, although these methods are better in corrosion resistance than quenched formed items of iron without plating, but not as sufficient in corrosion resistance as formed items of a plated steel sheet formed in an ordinary cold process. A cause of this corrosion resistance degradation has been estimated, as a result of studies by the present inventors, to be that Zn valatilizes, reducing the amount of plating (plating weight), and further more, the plating layer has an Fe—Zn alloy phase that is made up mainly of Fe that is solid-dissolved in Zn, so that the rust expansion of corrosion becomes large, and therefore accelerates the corrosion. Apart from the problem of plated steel sheets formed by the foregoing hot processing, aluminum plated steel sheets are employed for uses that require as high corrosion resistance as in the case of ordinary plated steel sheets. However, in the case where the aluminum plated steel sheet is used, the corrosion resistance after quenching becomes lower than that of cold-formed items made of a plated material.
Besides, Japanese Patent Application Publication No. 2000-248338 (JP-A-2000-248338) discloses a method in which a zinc or zinc alloy plated steel sheet, after being processed, is partially hardened by heating necessary portions at high frequency and then rapidly cooling the heated portions. However, since the heating after the processing causes strain, the shape of the component parts cannot be maintained, and therefore the method is not practical. Japanese Patent Application Publication No. 2006-022395 (JP-A-2006-022395) discloses a high-strength formed article excellent in corrosion resistance in which a phase made up of 30% or less by mass of Fe is contained in an amount of 30 g/m2 or more, and a manufacturing method for the high-strength formed article. However, since this method makes the entire formed article high in strength, there is a possibility of the processability and operability declining after the high strength is obtained. For example, as for the processing, the boring process becomes difficult to perform, and it is possible that a finishing process for preventing cracks can be needed. Besides, as for the operation, since the entire formed article, including its flanges, has high hardness, the spot welding in an assembly process of a motor vehicle fails due to undesired contact of a welding electrode, resulting in a problem of good welding quality being impossible to secure, or the like.
In view of the forgoing problems, there is a strong demand for a technology that makes it possible that a high-strength formed article that is good in processability and operability can be formed by a zinc or zinc alloy-plated quenched material that is superior in terms of corrosion resistance and cost.